Gonadotropin and activin enhance maturational com1395GONADOTROPINS

In most teleosts, 17a,20b-dihydroxy-4-pregnen-3-one (DHP) serves as the most potent maturation-inducing steroid (MIS) to initiate final oocyte maturation. The maturational competence or the responsiveness of oocytes to DHP increases when the ovarian follicles approach the final stage of growth. In the ze-brafish, we demonstrated in the present study that full-grown oocytes (;0.7 mm) exhibited the highest maturational compe-tence, which diminished progressively with decreasing size of the follicles. Using midvitellogenic follicles (0.49–0.56 mm), which had little response to DHP, as the material, the present study aimed at investigating the endocrine and paracrine mech-anisms that regulate maturational competence of the oocytes. In agreement with the results of studies in other teleost fish, pretreatment of follicles with gonadotropin (hCG) significantl